Improved high-gradient performance for medium-velocity superconducting half-wave resonators: Surface preparation and trapped flux mitigation

A development effort to improve the performance of superconducting radio-frequency half-wave resonators (SRF HWRs) is underway at the Facility for Rare Isotope Beams (FRIB), where 220 such resonators are in operation. Our goal was to achieve an intrinsic quality factor ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msub> <a:mi>Q</a:mi> <a:mn>0</a:mn> </a:msub> </a:math> ) of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mo>≥</c:mo> <c:mn>2</c:mn> <c:mo>×</c:mo> <c:msup> <c:mn>10</c:mn> <c:mn>10</c:mn> </c:msup> </c:math> at an accelerating gradient ( <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msub> <e:mi>E</e:mi> <e:mi>acc</e:mi> </e:msub> </e:math> ) of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mrow> <g:mn>12</g:mn> <g:mtext> </g:mtext> <g:mtext> </g:mtext> <g:mi>MV</g:mi> <g:mo>/</g:mo> <g:mi mathvariant="normal">m</g:mi> </g:mrow> </g:math> . FRIB production resonators were prepared with buffered chemical polishing. First trials of electropolishing (EP) and post-EP low-temperature baking of FRIB HWRs allowed us to reach higher gradient (15 MV/m, limited by quench) with a higher quality factor at high gradient, but <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:msub> <j:mi>Q</j:mi> <j:mn>0</j:mn> </j:msub> </j:math> was still below our goal. Trapped magnetic flux during the Dewar test was found to be a source of <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:msub> <l:mi>Q</l:mi> <l:mn>0</l:mn> </l:msub> </l:math> reduction. Three strategies were used to reduce the trapped flux: (i) adding a local magnetic shield (LMGS) to supplement the “global” magnetic shield around the Dewar for reduction of the ambient magnetic field; (ii) performing a “uniform cooldown” (UC) to reduce the thermoelectric currents; and (iii) using a compensation coil to further reduce the ambient field with active field cancellation (AFC). The LMGS improved the <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:msub> <n:mi>Q</n:mi> <n:mn>0</n:mn> </n:msub> </n:math> , but not enough to reach our goal. With UC and AFC, we exceeded our goal, reaching <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"> <p:msub> <p:mi>Q</p:mi> <p:mn>0</p:mn> </p:msub> <p:mo>=</p:mo> <p:mn>2.8</p:mn> <p:mo>×</p:mo> <p:msup> <p:mn>10</p:mn> <p:mn>10</p:mn> </p:msup> </p:math> at <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"> <r:mrow> <r:msub> <r:mrow> <r:mi>E</r:mi> </r:mrow> <r:mrow> <r:mi>acc</r:mi> </r:mrow> </r:msub> <r:mo>=</r:mo> <r:mn>12</r:mn> <r:mtext> </r:mtext> <r:mi>MV</r:mi> <r:mo>/</r:mo> <r:mi mathvariant="normal">m</r:mi> </r:mrow> </r:math> .